1. Field of the Invention
The present invention relates to a printing apparatus, feeding apparatus, and feeding control method. Particularly, the present invention relates to a printing apparatus and a feeding control method which supply a printing medium such as a printing paper sheet and print on the printing medium by causing a printhead to discharge ink.
2. Description of the Related Art
Many of recent printing apparatuses employ a DC motor serving as a driving source and servo control that allows high-speed driving and accurate position control by feeding back position detection information obtained by an encoder.
Especially, control using a DC motor enables high-speed rotation without step out, unlike control using a pulse motor. Motor position information can be detected accurately by using an encoder signal. Since the detection information is fed back to the motor control rule, accurate speed control and positioning with respect to a target position can be performed.
For example, Japanese Patent No. 3352612 discloses such a technique.
In this prior art, a target position that progressively increases to a final target position is generated in every servo period. However, if the load on the mechanism of a printing apparatus largely varies during servo control, the speed also largely varies.
FIG. 10 is a graph showing an example of time variations in the target position and detected position.
As shown in FIG. 10, a target position that increases by ΔP in every servo period (ΔT) is given. When the load on the mechanism of the printing apparatus increases at time T=T1, the PWM output signal of servo control continuously takes an upper limit value (PWM_MAX). In this case, the detected position (actual position) is unable to track the target position and greatly deviates from the target position. The load is eliminated at time T=T2. Then, the detected speed becomes much higher than the target speed to recover the position deviation. When the detected speed is high, deceleration is started by servo control to ensure a speed close to the target speed.
FIG. 11 is a graph showing comparison between a target position profile and a detected position profile and comparison between a target speed profile and a detected speed profile. Times T=T1 and T=T2 indicate the time when the load on the mechanism starts increasing and the time when the heavy load is eliminated, respectively, as in FIG. 10.
As shown in FIG. 11, at time T=T2, the heavy load is eliminated, and the detected speed increases to recover the delay in position detection. Even when deceleration starts abruptly, the correct final target position cannot be obtained.